I did consider that option, that GM didn't actually set eyes on MM but didn't want to admit it, but a few things has put me off that.
First, although he would not want to admit it, would he prefer to potentially jeopardise his daughter being found by restricting the abduction window the police would investigate? What happens if the culprit is found and it gets confirmed that MM was taken before GM's check?
The other thing that puts me off it is that the last of the Tapas 7 didn't arrive at the table until around 9 o'clock, just before GM's chceck and he and Kate had already been at the Tapas for 30 mins at this point. If the abduction was planned, and someone was watching the movements, you'd think they'd wait until all of the group were at the restaraunt before making a move. Otherwise there'd be every chance of bumping into one of the other group with MM. As the last of the group arrived (DP, FP and DW), they crossed paths with MO doing his first check. So again, if someone was watching the group, they would not act quite yet either. When MO returned he tells GM he listened by MM's shutter and all was quiet (shutters obviously closed at this point). GM decides he will still go and check inside the apartment anyway and leaves as MO settles back down at the table.
It "might" be possible that the intruder entered 5A just after MO returned from his chceck and before GM. And then got stuck in the apartment as GM entered and so hid somewhere (maybe behind the door). But I do think MM was still there when GM checked. JMO.